Question About Propane Pigtail ?

If I understand them correctly, a pigtail won't flow if connected to the bottle and the other end unconnected. I believe they need system back pressure to hold an internal valve open? But not 100% sure on that.

The best test would be to put the suspected bad pigtail on the known good side of the regulator with a known full bottle.

Then put the known good pigtail and known full bottle on the side of the regulator that didn't work.

If the suspected bad pigtail flows on the suspected side of the regulator, then probably a regulator problem.

If the good pigtail flows on the suspected bad side of the regulator, then regulator is probably good.

Then I would be quite sure it's a bad pigtail.

On the other hand, they're relatively inexpensive so preventative maintenance might dictate to replace them both. I did just that a couple weeks ago, when I had a pigtail begin to leak at the crimp joint.

How old? I would probably get a new regulator and new pigtails if more than 10 years old.
Otherwise the one pigtail is fine.
If you have a choice I recommend Cavagna brand (amazon)

The pigtails have a safety valve in them, that if there is a major leak, it will restrict the flow of the gas. Maybe a small gas source will work, but not a big one like a furnace or water heater. To correct problem try turning the gas off on both tanks, turn on the stove until it goes out. Then SLOWLY turn the gas tank on to reset safety valves, and see if that works. Could be one of the pigtails valves is more sensitive then the other.
On my last trip this September, I learned this the hard way. I even tried swapping tanks like you did.

I spent hours trying to figure out just how my propane system worked..It was 3 weeks ago my one 30# tank ran dry and I didn't leave the other valve open..So when I opened the valve the same thing happened to me, the indicator stay Red. I swapped tanks and still No Propane from my other 30# tank which is full...Ok after 3.5hrs cking lines and valves for blockage I noticed the BB and spring inside the ends of hoses and valves which are for leaks or rupture lines, they work by equalized pressure to release the propane..How it releases propane while the stove and water heater are on gets me..But if you open the valve on the tank slowly then close it then repeat about 4 to 6 times the line becomes pressurized.. Then light the burner on the stove to start the propane flow, if the flame goes out then repeat the slow valve opening and closing..After 20mins All was fine..Air? or stuck BB? Well I now have a extra Propane selector Valve in my tool box.

Propane will not spit out the open hose. When I pull a tank out to fill it, I never turn the other side off. The design is so that you can pull and swap a tank without taking the system off line.

Also, when one tank goes empty, the red indicator is supposed to display. You need both tanks full for a green indicator. Red tells you ONE tank is empty. Although it is possible when swapping tanks around like you did that the green indicator would pop up. But I don't think it will stay that way for long.

I suggest you let the automatic change-over valve work as it was intended. I know some guys won't, and insist on leaving a bottle turned off. But there is no good reason to do that IMHO.

With pigtail connected (ACME GREEN NUT screwed onto cylinder) other end disconnected from regulator you will get a little bit of propane flow out of end of hose.

The 'Excess Low Device' is inside the ACME GREEN NUT.
It is a spring loaded ball bearing that will limit the flow but not completely stop it.

W/O pigtail connected to cylinder NO flow will come out of cylinder if you open the OPD Service Valve. There is a spring loaded shut off valve that has to be held open with the 'stub' that is in end of the ACME GREEN NUT.
And if the ACME GREEN NUT is not fully connected...stub will not open the OPD shutoff valve.
And if stub has moved back due to thermal bushing failure in ACME GREEN NUT then OPD Shutoff valve will not open

Most flow issue can be a result of bad pigtail hose

............OK friends , I now realize that there are more safety FEATURES built into these items than I had realized ! I'm going to reinstall the one pigtail I removed and see IF the system will rebalance itself .
............I'm also going to order a set of new pigtails and auto change over valve . Both of my current pigtails as well as the change over valve are less than a year old ! From your posts It is clear to me that these components can be almost new and still malfunction . So , I'm going to keep a complete set of propane components ready for any problems that might occur .
............I just want to tell you'll I appreciate very much each post explaining how the various pieces function ! , jf

Once again, Old biscuit is gnat's-ass on the money.

These days just about every fitting in you propane system has a flow restrictor in it. The green (or other color for different flow rates) Acme fitting, just about every spud (the pointy part of a fitting or hose end) and just about any other fitting in your system. These DO GO BAD. Two choices--- either replace the bad piece, again and again and again or fix it. If you actually have 2 working brain cells (and seriously, no insult intended) you can drill out these restrictors and you'll have a system that now works and won't fail again. The down side is that you need to shut off valves when changing tanks, loosening and tightening fittings and you can't use your cigarette lighter for finding leaks. Of course, anyone who wants to RV and keeps his rig in reasonable condition DOES have those 2 working brain cells and many more, and loss of these "protect you from you" pieces won't bother you at all. For the other 1 of a million idiots out there, the government has got your back, regardless of how much the rest of us need to spend or deal with problems that should never have happened.